Speech privacy system



Aug. 27, 194s.

7 w. A. `MaNAlR SPEECH PRIVACY SYSTEM Filed Oct. 8`, 1941 -2 SheetsV-Sheet 1 Filed Orb. 8. 1941 2 Sheets-.Sheet 2 hmmm@ w .mi

AZTORNEV Patented Aug. 27, 1946 SPEECH PRIVACY SYSTEM Walter A. MacNair, Summit, N. J., assignor to Bell Telephone Laboratories, Incorporated, New York, N. Y., a corporation of New York Application October 8, 1941, Serial No. 414,054

4 Claims.

The present invention relates to coordinating the movements of mechanisms such as rotary distributors used in electrical communication.

The invention has particular reference to, and will be disclosed as embodied in, a speech privacy system in which rotary distributors are used to break the speech waves up into fragments and send them out in changed sequence, and conversely to receive the transmitted wave fragments and rearrange them into understandable speech.

The invention will be disclosed as embodied in a system using start-stop distributors in which the brushes are released once each revolution by a start impulse and progress through the revolution in unison but independently driven until, at the end of the revolution, they are stopped in readiness to be released again simultaneously by the start impulse.

It is common practice in printing telegraphy to employ start-stop commutators in which the receiving distributor is normally driven slightly faster than the transmitting distributor, which means that under normal conditions it reaches its stop sooner than the transmitting distributor and therefore remains stationary for a slightly longer period than the transmitting distributor. This is done in order to insure that under abnormal conditions that might tend to slow up the receiving distributor it will still be able to reachv the stop member in time to be released by theV start impulse. This practice is permissible in telegraphy where a small portion of a transmitted impulse is suliicient to operate the receiving relay and it does not matter if some of the impulse is lost. In fact, it is common practice to make the receiving commutator segments shorter than the transmitter segments with a view to picking out the middle portion of the impulse.

In speech transmission, however, in which the speech is recorded on a suitable medium such as i a moving magnetic tape and subsequently distributed in segments it is essential for full intelligibility that the segments of the recorded mes- Sage be preserved in their entirety or nearly so, and this imposes a much greater restriction on the synchronizing problem than in the case of telegraphy.

A further difficulty presents itself where twoway speech transmission is to be carried out with the same distributor serving alternatively as transmitting and as receiving distributor. `In such case it would be impractical to drive the receiving distributor at a faster rate since the same distributor is, an instant later, to be changed over to transmitting.

i the same length of time.

In accordance with the present invention the distributors are al1 driven as nearly as practicable at the same constant speed and are stopped at stated intervals, such as once each revolution, and started in synchronism and phase with one another by a start impulse sent out from the transmitting distributor. In order to allow for small errors in speed the impulse is delayed slightly to permit all receiving distributors that may be slow, to within an operative margin, to arrive at their stop positions before the start impulse is sent. This operation periodically corrects for small errors in speed, whether representing too fast or too slow speed, in the receiving distributors.

When a receiving distributor is switched over to become a transmitting distributor it automatically sends out the start impulses and all receiving distributors are thus synchronized to it.

It wi11 be noted that this accomplishes something of the same result as in the case of telegraph systems with receiving distributors set to run slightly faster than the transmitting distributor but avoids the troublesome disturbance that would be produced by that type of operation in a speech transmission system. That is, if the receiving distributor were normally set to run slightly faster, it would gradually get more and more out of phase with the transmitting distributor during the complete revolution, which would mean that in each successive commutating interval a small piece of the speech fragment as transmitted would be lost while a small piece of the next successive speech fragment would be picked up out of its proper sequence. This would result in loss of intelligibility and increase of noise, and this would occur when the receiving distributors were running in their normal or intended manner. In the system according to the invention, however, there is no such loss of intelligibility or increase of noise so long as the distributors are functioning normally since they are started in unison and travel at the same speed throughout their entire revolution. A11 distributors are, under normal conditions, held stopped A fast receiving distributor is stopped for a slightly longer time and a slow receiving distributor is stopped for a shorter time than normal.

The nature of the invention and its various objects and features will appear more fully from the following detailed description in connection with the accompanying drawings in which:

Figs. 1 and 2 when placed side by side with Fig. l at th'e left show in schematic circuit diagram the two terminals of a two-way privacy telephone .s system incorporating the invention, while Fig. 3 is a partial diagram of a radio terminal which may be substituted for the portion of Fig. 1 to the right of the line X-X, and Fig. 4 is a similar partial diagram to be substituted for the portion of Fig. 2 to the left of the broken line Y-Y.

Referring to Fig. 1, a constant speed motor I of suitable type drives a shaft I I which' is shown as driving magnetic tapeV l2 mounted on the rim of a suitable disc, brush arm I3 rotatable over the segments of commutator 29, and a gear train I 4, I5 driving a contact making arm I6. The system is arranged to transmit with privacy when speech is spoken into the transmitter I1, under certain circuit conditions to be described,v and to receive with privacy in receiver IB. The transmission and reception of the privacy waves takes place over the line 22 shown interconnecting the terminals in Figs. l and 2. Y A number of two-position switches are illustrated at 23, 24, 25, 25 and 21. vThese switches may,A if preferred, be ganged together or they may be inthe formof relays operated by a single push. button. These switches are all assumed to be operated to their upper positions in the figure (and switch 21 to the right) when speech is to be transmitted and to be operated to their lower or Opposite position when speech is to be received. Such switches are commonly referred to as push-to-talk switches and may conveniently be retracted to their receiving positions by.

springs (not shown), the receiving position in` such case being considered normal. The circuit of Fig. lwill first be described for the transmitting conditions so that switches 23 to 21 are considered thrown to their upperor talking positions.y

The tape I2 is assumed to be rotating in a clockwise direction at constant speed. Located around the periphery of the tape are an erasing magnet 2l supplied continuously with current from the battery shown, a recording magnet I9 andnine reproducer magnets 20 located at equal intervalszaround the tape. When speech is spoken intov transmitter I1, therefore, the speech is recorded on the tape at I9 and the recorded portions in passing the magnets 20 generate corresponding electromotive forces in their wind-` ings. Theterminals of these windings are connected through code switches 38, five in number, and thence through contacts of switch .21 to the segments of Vcommutator ring 29, which are numbered in order I, 2, 3, 4, 5 over one half ofthe circumference, withthese numbers repeated in the second half of the circumference, diametrically opposite segments being directly connected together. The code switches are so arranged that one winding terminal of reproducers 2i) may be connected in a number of different sequences to the commutator segments of ring 29. Brush arm I3 carries brushes bridging across from segmented ring 29 to solid ring 30, the' latter being connected to one terminal of winding of speech coil 3|, the oDDOsite terminal being common to one terminal of .each of the reproducer magnets 20.

It will be seen, therefore, that as the brush arm I3 rotates and sweeps .the brush over the segments of the commutator, different reproducers 20 lare connected to the speech coil 3| in sequence, the particular sequence is at .any given time determined by the setting of the code switches 38.V For example, designating the reproducers 2i) beginning next to the recording magnet I9 and continuing around the periphery IJII 4 of the tape as reproducers a, b, c, i, reproducer a may with one code setting be connected to commutator segment I, reproducer c may be connected to segment 2, reproducer e may be connected to segment 3, etc. With a different setting of the code switches 33 reproducer d might be connected to segment I and other reproducers to the other segments. In this way it is arranged that the currents transmitted `through coil 3l represent fragments of the recorded speech picked up in a sequence different from th'e sequence in which they were recorded. Moreover, this sequence is varied from time to time by changing the settings of the code switches 38.Y vThis general method of transmitting fragments of speech out of their normal sequence and of changing the code is more fully disclosed in a, copending application of J. C. Steinberg, Serial No. 401,897, filed July 1l, 1941, which may be consulted for further details including the construction of th'e code switchmechanism 38. v The speech waves occurring out of their normal order in coil 3l are transmitted through switches 25 and 24.` and band-pass iilter 36 to the outgoing line 22.

Attention will now be given to the start-stop mechanism to which the invention particularly relates. YThe brush arm I3 frictionally engages the shaft II by suitable friction clutch mechanism sok that when the brush is released by latch 32 it rotates with the shaft Il but can be held stationary by latch 32 while shaft ll.continues to rotate. Latch 32 is released by tripping magnet 33 whenr energized from source 34 under control of contacts 35 when the station is transmitting or under the control of similar impulses received over the line from a distant stationwhen the station shown is receiving.

The gear train I4, I5 drives the arm I6 at a slightly slower rotational speed than shaft II. For example, the differences in speed may be 2 per cent. In this case gear I4 Vhas forty-nine teeth while gear I5 has fifty teeth.

Assuming that arm I3 is stopped against latch 32 andthat cam I6 is about to pass under and lift the lower spring of contact 35, this contact will, then, close and permit the flow of current from generator 34 through contact 35, switch 26, transformer 41, through the filter 31 to the opposite terminal of source 34. The impulse transmitted through transformer 41 is rectied at 46 and operates tripping magnet 33 to release brush arm I3. The tripping magnet 33 is energized releasing brush I3Y and simultaneously the start impulse is sent out through filter 31 to line 22 to the distant station to release the brush arm thereat. The generator 34 may generate a frequency of 2000 cycles per second byway of example. In this case lter 31 is a narrow bandpass iilter passing 2000 cycles'while filter 36 is a band suppression filterofering high attenuation to .2000 cycles but freely passing frequencies in the speech band both lower rand higher than 2000 cycles.r Y

Referring now to Fig. 2, it will be-noted that this figure is a duplicate of Fig. 1 with corresponding parts indicated by the same. reference.

numerals primed. Continuing the description of the operation of transmitting. from the station of Fig. 1, it is assumedthat all of the switches 23' to 21 are in their loweror receiving positions,

When vthe synchronizing impulse was sent outv as described from the station of Fig. 1 it was received over line 22 at the station shown in Fig. 2 and selectively transmitted through filter 31',

lower contact of. switch 26', transformer 41', rectifier 4E and winding of tripping magnet 33', thus releasing brush arm i3 simultaneously with the release of brush arm 3. As long as` the station of Fig. 1 continues to transmit, switch 35 sends out a synchronizing impulse once each revolution of the cam l5 which as stated rotates at a slightly lower speed than shaft Il. This causes the brush arm i3 to be stopped momentarily once each revolution. As stated, brush I3', if running in exact synchronism, is stopped once each revolution for the same length of time or if not in exact synchronism it is stopped for a shorter or longer length of time depending upon whether the brush is running slow Yor fast by a slight amount.

The jumbled speech waves received through filter 35' and switches 2e and 23 energize the winding of recording magnet I9 to make a record of the received waves on the tape I2. As this tape rotates past the various reproducer magnets these latter pick up the recorded waves and transmit them through code switch 33 and switch 21 to the segments of commutator 29' and the wiring and settings of the code switches are such that as the brush I3' passes over the commutator segments in succession the speech fragments are rearranged in their normal order in the coil 3l', from which they are transmitted through switch 25 to receiver I8'. When the code switches 3B and the wiring are made in accordance with the disclosure of the Steinberg application above referred to, a given setting of the knobs 28 will result in transmitting in accordance with a particular code and also in receiving in accordance with the same code. The knobs 28 and 28 in Figs. 1 and 2 are, therefore, given the same setting.

When the station in Fig. 2 desires to change from receiving to talking, the switches 23 to 25 are shifted to their talking position, while the corresponding switches of Fig. 1 are shifted to their receiving contacts. The station of Fig. 2 then becomes the transmitting station and it will be noted that switch 26 now controls the sending out of the synchronizing pulses from switch springs under control of cam I These synchronizing pulses when received at the opposite station, Fig. 1, pass through iilter 31 and switch 25 in its lower position, coil 41 and rectifier 46 to the tripping magnet 33, the switch 35 being under these conditions ineffective on account of the break in the circuit at switch 26.

In the alternative terminal shown in Fig. 3, the transmission is on a four-wire basis and the transmitting and receiving channels are radio channels although they could as well be line wire channels operating at carrier or voice frequency. Switch 24 (Fig. 1) is not needed but when the switches 23, 25 and 26 are thrown to talking position, the output of coil 3l is impressed on band suppression filter 39 (similar to 36) leading to radio transmitter 44 of known and suitable type for transmitting speech modulated waves. In this case the waves are modulated by the jumbled speech. The synchronizing impulses are transmitted as before from source 34, but in this case through switch spring 43, band-pass lter 4l (similar to 31), switch spring 42 and across the input terminals of transmitter 44.

When the switches are in receiving position, the received modulated waves from the distant station are demodulated to the 'speech frequency level in radio receiver 45 and are sent through band suppression filter 4i) (similar to 36) whence theyV pass into coil 3l for decoding as previously described. The received synchronizing impulses from receiver 1&5 `pass through switch springs 4.2 and 43, lter M, and to the tripping magnet as previously described. The construction and operation of Fig. 4 is apparent from the foregoing description, similar reference characters being used in Fig. 4 with primes.

Amplication will ordinarily be introduced at various points in both the transmitting and receiving circuits for both the message waves and synchronizing waves, but amplifiers have been omitted from the drawings in the interest of simplicity.

The invention is not to be construed aslimited to the detailed disclosure but may be varied to suit conditions within the scope of the claims, which follow.

What is claimed is:

1. A two-way signal terminal comprising means to record signals for transmission and means to record received signals, distributor means for breaking up the recorded signal before transmismission to render transmission private, said distributor means operating to restore received broken up signals to normal for intelligible reception, start-stop mechanism for said distributor including a latch tripping magnet, means coordinated with said distributor for energizing said tripping magnet and simultaneously sending out a start impulse once each revolution of said distributor means, and means to delay the energizing of said tripping magnet and the sending of said start impulse relative to the movement of said recording means.

2. In a signalling systemy means to record signals for transmission, transmitting distributor means for breaking up the recorded signal-s, before transmission, distantly located means for recording the transmitted signals and receiving distributor-means for restoring the signals to a form suitable for reception, start-stop mechanisrng for both of said distributor means, means operatively associated with the transmitting distributor means for periodically starting the same and for simultaneously sending out a start impulse to cause starting of the receiving distributor means, and means to delay the starting of said transmitting distributor means and the simultaneous sending out of said Start impulse until after the transmitting distributor means has been stopped.

3. A two-way station for a privacy system for signals such as speech, including a continuously operating recorder for recording normal signals to be transmitted and secret signals to be received, a rotary distributor for breaking up the recorded signals and rearranging them in abnormal order for sending and normal order for receiving, start-stop mechanism for said distributor, switches for connecting the recorder and distributor to a transmission path in either transmitting or receiving condition with respect thereto, a continuously driven contact closer and means controlled thereby for operating said startstop mechanism to release the distributor for rotation and simultaneously to tran-smit a start impulse to said transmission path when said station is in transmitting condition, and means to drive said contact closer at a slightly reduced speed relative to said recorder to provide a Small normal stop interval for said distributor.

4. In a privacy system for signals such as speech, continuously running recorders at each station for the signals, means for driving said re- 7 corders at the same nominal speed, start-stop distributors for breaking the recorded signals into fragments on a time basis and effecting dierf ent relative delays in the sending and receiving of the various fragments, switches for conditioning each station to either transmit or receive, means in each station operative when the station is in transmittingeondition for starting its own distributor and for sending a start impulse to the other stations once each revolution of its distributor, and means to delay by a slight amount the sending of said start impulse relative to the movement of the recorder of the station transmitting to provide a short interval in which each distributor at the station or stations receiving may arrive in proper starting phase before receipt of the start impulse. Y

WALTER. A. MACNAIR. 

